This invention relates in general to electrical machines and in particular to an improved method for operating a switched reluctance electrical generator in an optimum manner to achieve high efficiency.
Electrical machines can be generally categorized as being either motors, which convert electrical energy to mechanical energy (rotary mechanical energy, for example), or generators, which convert mechanical energy to electrical energy. Generally speaking, electrical motors convert electrical energy to mechanical energy by establishing and controlling electromagnetic fields so as to cause the desired mechanical motion. Electrical generators, on the other hand, are generally responsive to the mechanical motion in the presence of these electromagnetic fields so as to cause the flow of electrical current. There are many different types of electrical machines, and the operating characteristics of such electrical motors and generators vary widely from type to type.
A reluctance machine is an electrical machine that produces torque as a result of the tendency of its movable part (typically referred to as a rotor) to move relative to its stationary part (typically referred to as a stator) in such a manner that the reluctance of a magnetic circuit between the rotor and the stator is minimized. Usually, the stator is provided with a plurality of opposed pairs of inwardly extending or salient poles, while the rotor is provided with a different plurality of opposed pairs of outwardly extending salient poles. Windings provided on the stator poles are selectively energized to create a rotating magnetic field, which attracts the rotor poles toward the stator poles and causes the rotor to rotate relative to the stator. In one type of reluctance machine, the energization of the phase windings occurs at a controlled frequency. This is generally referred to as a synchronous reluctance machine, which may be operated as either a motor or a generator. In a second type of reluctance machine, circuitry is provided for detecting the angular position of the rotor and for energizing the phase windings as a function of the position of the rotor relative to the stator. This is generally referred to as a switched reluctance machine (or, in some instances, a variable reluctance machine), which may also be operated either as a motor or a generator.
Switched reluctance machines, operated either as motors or generators, are receiving increased attention with the improvements of power electronics technology and machine design. Some of the advantages of switched reluctance machines include the absence of windings or permanent magnets on the rotor, durability, the lack of brushes or other mechanical commutation structures, and the independence of the phases. These advantages can make the use of switched reluctance machines more efficient and less expensive in some applications.
The efficiency of any electrical machine (i.e., the ratio of the output mechanical energy generated by the motor to the input electrical energy supplied thereto, or vice versa in the case of a generator) is an important consideration in any application. However, when used in an automotive or other vehicular application, the efficiency of the electric motor is very important because it can have an impact on the fuel economy or battery life of the vehicle. Thus, it would be desirable to provide an improved method for operating an electrical machine, such as a switched reluctance electrical generator, in a manner that is highly efficient.
This invention relates to an improved method for operating a switched reluctance electrical generator in a manner that is highly efficient. Initially, a mapping technique is performed to obtain data relating to all of the possible operating conditions of the generator system that generate the desired output power. This mapping technique can be performed empirically or by computer simulation. Then, the effective phase currents supplied to the windings on the stator are measured or calculated. Next, the optimum conduction angles can be selected as those turn-on angles and turn-off angles that occur using the smallest effective phase currents supplied to the windings on the stator. Lastly, the generator system is operated using the selected optimum turn-on and turn-off angles. If desired, a feedback loop can be provided for comparing the actual output power that is generated by the generator system with a desired reference output power level to insure that such actual output power is maintained at or near the desired output power. To accomplish this, the conduction angles can be adjusted in response to such comparison to increase or decrease the actual output power as necessary.
Various objects and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings.